In many film applications, such as packaging of foodstuffs, chemical and hazardous materials and in medical applications, the industry requires films having certain properties. In the packaging of foodstuffs, for example, the films must have high puncture resistance, high clarity and gloss, and reduced permeability to gases and/or vapors. The films used to manufacture containers for chemicals and hazardous waste materials must have a high puncture resistance, high elongation strength, high tear resistance and chemical resistance. Films used in medical applications, such as blood bags, must have a high puncture resistance, low modulus, high tear resistance and auto-clavability.
Thus, there is a need for a polymer material which has a lower flexural modulus, good tear strength, higher elastic recovery, reduced draw resonance, as well as all of the other desirable properties.
Films made from ethylene polymers, homopolymers, e.g., HDPE and LLDPE, and copolymers, e.g., LLDPE, and propylene polymers, such as crystalline homopolymers of propylene and random copolymers of propylene and ethylene, do not provide such a combination of desirable properties.
Attempts have been made to overcome the shortcomings of these polymers by preparing heterophasic mixtures of crystalline propylene polymers and 8 to 25% of an elastomeric propylene-ethylene copolymer by sequential polymerization in the presence of a stereospecific Ziegler-Natta type catalyst. However, films of such heterophasic compositions are subject to the formation of fisheyes, inadequate tear strength or the formation of rough surfaces.